With the progress of the society and the development of science and technology, the thin film field effect transistor liquid crystal display (TFT-LCD) has become the mainstream product in the field of display nowadays, which plays a crucial role in industrial production and daily life, and has got more and more favour of people.
Liquid crystal alignment is a necessary link in production of liquid crystal panels. The rubbing technology is widely used in the current TFT-LED industry to rub the glass substrate, such that trenches of a certain angle are formed at the surface of polyimide molecules, the liquid crystal molecules are arranged along the trenches to achieve the purpose of alignment. The rubbing process is relatively simple and convenient, it is the current mainstream technology and has a lot of advantages, such as the process is mature, the rubbing alignment stability is good; it is convenient for implementation and the operation is easy. However, many disadvantages still exist currently: nonuniform rubbing results in nonuniformity of alignment of the liquid crystal molecules, which influences the display quality; when the rubbing cloth rubs the substrate, the surface of the substrate may be easily scratched, which may cause quality defects; many rubbing debris (such as polyimide debris, cloth hair debris) may be produced in the rubbing which may pollute the liquid crystal, and may cause display defects such as Mura etc.; the IPS-type liquid crystal screen has a relatively high requirement on the image quality, however, the rubbing process could hardly meet the requirements (such as relatively low contrast etc.) of the high pixel display; static electricity generated by rubbing may easily break down the display elements on the array substrate, which may result in bad display.
Therefore, a new liquid crystal alignment technology, i.e. optical alignment process, rises gradually in the industry, i.e., a method of initiating photopolymerization, photoisomerization or photodecomposition reaction to the optical alignment material film on the substrate through UV irradiation, so as to generate anisotropy of the surface, thereby inducing alignment of liquid crystal molecules, this method is a non-contact liquid crystal alignment technology and has overcome the shortcomings of the rubbing alignment. The optical alignment process adopts non-contact exposure, which would not damage the surface of the PI film and could prevent light leakage, such that the luminance in the dark state is lower, while the contrast in the bright state remains unchanged, thereby achieving the requirement of high contrast. The light decomposition materials are also divided into two types, one is small molecule type, which only needs to perform secondary curing process, however, thus produced products still have many defects which are difficult to be solved, such as Mura etc. The other is large molecule type, which needs to perform secondary cleaning, the Mura defect of the product is relatively small, which has a considerable prospect. However, as for the secondary cleaning process, there is not a successful method yet in the industry that can clean up the decomposed residual PI (polyimide) liquid molecules, which influences some performances such as contrast etc., of the product greatly. The defects of the optical alignment process also make the liquid crystal screen have related badness, such as serious afterimage of the display, bad reliability etc.